The present invention relates to a method and a device for controlling a brushless electric motor, preferably comprising a permanent magnet rotor and armature windings supplied with current from a DC-power supply via switching means controlled to selectively supply current to the armature windings.
In this field it is known to control the switching means using the zero crossing of the back electromotive forces developed during operation in the windings of the motor as a reference and delaying the commutation relative to the zero crossings in different ways.
From U.S. Pat No. 5,780,983 it is known to generate commutation signals for a brushless DC motor drive apparatus by comparing selected combinations of the terminal voltages of the armature windings. In order to adjust the timing of the commutation signals one combination of terminal voltages and an other combination of terminal voltages amplified with a certain gain factor are compared. This method requires a relatively complex set-up of amplifiers and comparators in order to generate the commutation signals.
From U.S. Pat. No. 5,640,073 it is known to use a delay of the back electromotive forces and to compare these delayed back electromotive forces with a saw-tooth wave reference voltage having a frequency proportional to the rotor speed and an amplitude whose center voltage is equal to the midpoint voltage. The amplitude of the saw-tooth wave is adjusted in accordance with the rotor speed or rotor current in order to adjust the resulting delay of the commutation signals. The object of the provision of the saw-tooth wave signal is to make it possible to use a delay angle for the back electromotive forces, which is less than 90xc2x0, in order to provide a quick response to sudden load variations. When the rotor speed is below a predetermined speed, it may be necessary to increase the time constant for the delay of the back electromotive forces.
From U.S. Pat. No. 5,506,487 a commutation method is known which does not use the detection of the zero-crossing. The three back electromotive forces are multiplexed onto one line and the polarity thereof is considered. A counter is counting up and down during periods with positive and negative multiplexed back-electromotive force respectively. The counter counts until a predetermined value is reached, whereafter commutation is performed. The predetermined value corresponds to the delay of the commutation after the zero-crossing of the back-electromotive force. This method is relatively complicated and requires calculating power.
The apparatus and method described in U.S. Pat. No. 5,327,053 relate to solving a different problem, namely the detection of rotor position prior to starting up the motor. The control of the motor after start up is based on a back-electromotive force zero-crossing commutation scheme which is not described in detail.
It is the object of the present invention to provide a method and a device of the kind mentioned above of a simple nature, with which it is possible to control the motor with a selfregulating commutation angle without delay filters or time measurements and without the use of the mid point voltage of the motor, and this object is achieved with a method of said kind which according to the present invention comprises the steps set forth in the characterizing clause of claim 1 and a device of said kind which according to the present invention comprises the features set forth in the characterizing clause of claim 6. With this arrangement, the generating of commutation signals is performed in a simple manner taking advantage of the trapezoidal curve form of the back-electromotive forces which are compared to a low and high reference voltage, whereby these reference voltages are adjusted to achieve the correct timing of the commutation signals. Furthermore, measurement of the zero-crossings of the back-electromotive forces is avoided which can be complicated due to electrical noise.
Preferred embodiments of the method and apparatus are revealed in the subordinate claims 2-5 and 7-10, primarily aiming at simplifying the generation of the reference voltages.